Equity Investments (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jul. 02, 2013 SIR	Dec. 31, 2013 SIR	Dec. 31, 2012 SIR	Mar. 15, 2013 GOV	Dec. 31, 2013 GOV	Dec. 31, 2012 GOV	Dec. 31, 2011 GOV	Dec. 31, 2013 AIC	Dec. 31, 2012 AIC	Dec. 31, 2011 AIC
Equity Investments
Equity method investments, ownership percentage				56.00%	44.20%	56.00%	18.20%	0.00%	18.20%		12.50%	25.00%
Equity Investments	$ 517,991	$ 184,711			$ 512,078				$ 173,452		$ 5,913	$ 11,259
Equity in Earnings	25,754	11,420	11,377		21,153			4,111	10,836	11,238	490	584	139
Equity investments, common shares owned				22,000,000	22,000,000		9,950,000
Equity investments, market value based on quoted market prices					588,280
Equity investments, quoted market price of common share (in dollars per share)					$ 26.74
Gross proceeds received on issuance or sale of common shares							250,740
Net proceeds received on issuance or sale of common shares							239,576
Common stock price per share (in dollars per share)				$ 28.25			$ 25.20
Ownership percentage after offering					44.20%
Market value of common shares exceeding carrying value				17,609
Amortization of the difference between carrying value and share of underlying equity				34 years
Gain on issuance of shares by an equity investee		7,246	11,177				66,293
Cash distributions				$ 19,800				$ 4,279	$ 16,816	$ 16,617
Issuance of shares, net (in shares)				10,500,000